Ongoing Deformation Processes Research Articles

НАБЛЮДЕНИЯ ЗА ДЕФОРМАЦИЯМИ СООРУЖЕНИЙ В ЗОНЕ СТРОИТЕЛЬСТВА МЕТРОПОЛИТЕНА Г. АЛМАТЫ

The article presents the results of monitoring the condition of surface buildings and structures during the construction and operation of the Almaty metro located within a radius of 80 meters. In such a situation, the problem of managing geodynamic processes during the construction and operation of the subway can be solved on the basis of the methodology of conducting geodetic monitoring of the condition of engineering structures considered in this article. It is shown that when conducting geodetic monitoring and monitoring of deformations of engineering structures using modern geodetic instruments such as global navigation satellite systems, electronic total stations, laser levels, it will allow obtaining information about the ongoing deformation processes of buildings and structures located in close proximity to the metro under construction and in operation with a high degree of accuracy, which will ensure timely management decisions solutions for long-term and safe operation.

The February 2018 seismic swarm in São Miguel, Azores

The island of São Miguel is among the most seismically active areas of the Azores archipelago. This work focuses on the most significant recent swarm, which occurred on February 2018. We set up an automated procedure to process continuous full seismic waveform data from local stations to generate high-quality earthquake information on the volcano unrest episode. First, we applied an automated detector software, next we located the detected events and then classified the earthquakes based on their waveform similarity, identifying three families of seismic events. We then extended the catalog by template matching. Finally, we computed moment tensors to investigate the source mechanisms of the largest earthquakes. Our results image the ∼2-week swarm evolution. The activity started with a precursory phase with low rate and low magnitude (ML < 2.0) seismicity and the activation of a deeper structure (∼10-15 km). After ∼1 week, a new earthquake family emerged at shallower depths (∼8–12 km) reaching magnitudes up to ML 3.4. Finally, a third slightly shallower family was activated. Moment tensors show mostly normal faulting mechanisms, striking ∼NW-SE, compatible with the orientation of the regional stress field. A surface deformation transient was recorded by geodetic stations, starting with the swarm, and continuing over the following ∼17 months, corresponding to either inflation or extension around the swarm region. The prolonged surface deformation implies a process that was initiated during the swarm and subsequently accommodated mostly aseismically. We interpret the seismicity observed at the early stage of deformation as indicating episodic fluid injection through the crust, related to the local hydrothermal or magmatic systems. We conclude that the Fogo-Congro region continues to be seismo-volcanically active, with both seismic and aseismic deformation observed and requiring close multidisciplinary monitoring. The proposed methology based on the automated analysis of continuous waveform data provides high-quality imaging of the spatio-temporal evolution of seismicity, which can be used elsewhere in the operational monitoring of seismo-volcanic crises to gain insight into the ongoing deformation processes, improve hazard assessment and help in the development of effective mitigation strategies.

Numerical study of in situ acoustic emission monitoring for small-scale hydraulic fracturing

Abstract Reservoir monitoring is necessary to achieve safe hydrocarbon extraction. It requires monitoring small acoustic emission (AE) events, assisting in determining the exact location, extension and direction of potential damage as early as possible. However, microcracks cannot be detected by the microseismic monitoring networks due to the limitations of frequency range and sensitivity. In contrast, the in situ AE monitoring system extends the detection range to higher frequencies and can detect very small deformation processes with high resolution and sensitivity. It provides detailed insights into ongoing deformation processes. However, the receivers of an in situ AE monitoring system need to be very close to hydraulic fracturing experiments due to the fast decay of high-frequency signals. In this work, by constructing four in situ AE monitoring models, the imaging effects of the interferometric imaging method for three different kHz-level frequency sources at different distances and orientations relative to the monitoring well are investigated. The numerical results show that: the higher the source frequency, the higher the imaging resolution; when the vertical orientation coverage of the source by the monitoring system is incomplete, the closer the source to the borehole axis, the worse the imaging resolution and location accuracy; when the vertical orientation coverage of the source by the monitoring system is complete, the imaging resolution and location accuracy are both improved, especially when the vertical azimuthal coverage angle is large. The integrity of the orientation coverage of the source by the monitoring system plays a critical role in improving source location accuracy.

A 20 year-long GNSS solution across South-America with focus in Chile

Over the last 3 decades, GPS measurements have been instrumental in quantifying tectonic plates current motion and deformation. Complex patterns of deformation along the plate boundaries revealed heterogeneous coupling on the plates interface and imaged seismic segments at different stages of their seismic cycle. Along the South-American trench in Chile, where large earthquakes occur frequently, continuous GPS observations (cGPS) captured both the long-term plate motion and the transient deformations associated with the seismic cycle. Over the years, a network of hundreds of cGPS stations has been deployed all across the South-American continent by many different institutions for many different sorts of purposes ranging from geographic reference to tsunami early warning. We report here on the processing of 20 years (2000–2020) worth of data over a selection of cGPS stations, devoted to the quantification and analysis of the deformation along the Chilean subduction zone between 18°S and 40°S. We use all available data near the trench in Chile and a less dense network inside the continent where the gradient of deformation is lesser. Our database, namedSOAM_GNSS_solENS, provides time series of precise daily station position, obtained from double difference (DD) processing and expressed in the International Terrestrial Reference Frame 2014 (ITRF14). These time series allow to quantify, with sub-millimetric precision, any kind of ongoing deformation process, either from tectonic origin such as interseismic deformation, co- and post-seismic displacements associated with earthquakes, transient deformation associated to seismic swarms and/or a-seismic slow-slip events, or of other origin such as hydrological loading (for ex., the Amazonian basin load) or any other type of loading affecting the surface of the Earth (tides, atmosphere, etc.). We also provide a database of coseismic displacements associated with close to 60 earthquakes of Mw larger than 6.5 that occurred over the last 20 years within the observation area. All time series are directly accessible through a deposit and we plan to make them available through a web interface that will allow any user to perform elementary operations like estimating offsets, detecting outliers, detrending, filtering and stacking. That database will evolve with time, aggregating more data. In the future, we also plan to complement that database with a rapid solution in quasi real time processed in Precise Point Positioning (PPP), and with hourly atmospheric delays associated with water vapor content of the lower layer of the atmosphere.

Inflation-Deflation Episodes in the Hengill and Hrómundartindur Volcanic Complexes, SW Iceland

Non-eruptive uplift and subsidence episodes remain a challenge for monitoring and hazard assessments in active volcanic systems worldwide. Sources of such deformation may relate to processes such as magma inflow and outflow, motion and phase changes of hydrothermal fluids or magma volatiles, heat transfer from magmatic bodies and heat-mining from geothermal extraction. The Hengill area, in southwest Iceland, hosts two active volcanic systems, Hengill and Hrómundartindur, and two high-temperature geothermal power plants, Hellisheiði and Nesjavellir. Using a combination of geodetic data sets (GNSS and InSAR; Global Navigation Satellite Systems and Interferometry Synthetic Aperture Radar, respectively) and a non-linear inversion scheme to estimate the optimal analytical model parameters, we investigate the ground deformation between 2017–2018. Due to other ongoing deformation processes in the area, such as plate motion, subsidence in the two geothermal production fields, and deep-seated source of contraction since 2006, we estimate 2017–2018 difference velocities by subtracting background deformation, determined from data spanning 2015–2017 (InSAR) or 2009–2017 (GNSS). This method highlights changes in ground deformation observed in 2017–2018 compared to prior years: uplift signal of ∼10 km diameter located in the eastern part of the Hengill area, and geothermal production-related temporal changes in deformation near Húsmúli, in the western part of the Hengill area. We find an inflation source located between the Hengill and Hrómundartindur volcanic complexes, lasting for ∼5 months, with a maximum uplift of ∼12 mm. Our model inversions give a source at depth of ∼6–7 km, located approximately in the same crustal volume as an inferred contracting source in 2006–2017, within the local brittle-ductile transition zone. No significant changes were observed in local seismicity, borehole temperatures and pressures during the uplift episode. These transient inflation and deflation sources are located ∼3 km NW from a source of non-eruptive uplift in the area (1993–1999). We consider possible magmatic and hydrothermal processes as the causes for these inflation-deflation episodes and conclude that further geophysical and geological studies are needed to better understand such episodes.

An Application of Persistent Scatterer Interferometry (PSI) Technique for Infrastructure Monitoring

In the absence of systematic structural monitoring to support adequate maintenance standards, many existing infrastructures may reach unacceptable quality levels during their life cycle, resulting in significant damage and even potential failure. The metropolitan area of the Gulf of Salerno (Italy), served by a complex multimodal transport network connecting the port area to the roads and railways surrounding the urban area, represents an important industrial and commercial hub at the local and international scale. This particular scenario, developed in a complex morphological and geological context, has led to the interference and overlapping of the transport network (highway, railway, main and secondary roads) that run through the piedmont area north of the port. Given the relevance of the area, our research aims to highlight the capabilities of the persistent scatterer interferometry (PSI) technique, belonging to the group of differential interferometric synthetic aperture radar (SAR), to extract space–temporal series of displacements on ground points or artifacts with millimeter accuracy useful to understand ongoing deformation processes. By using archived data from the European Space Agency missions, i.e., ERS1/2 (European remote-sensing satellite) and ENVISAT (environmental satellite), and the most recent data from COSMO-SkyMed constellations, it was possible to collect a 28-year dataset that was used to spatially analyze displacement patterns at a site-specific scale to check the stability of viaducts and embankments, and on a larger scale to understand the activity of the surrounding slopes. Despite the different resolution and subsequently the ground density, the analysis of the different datasets showed a spatiotemporal consistency in the displacement patterns that concerned two subareas showing significant annual velocity trends, one northeast of the city and the second in the port area. The analysis presented in this paper highlights how a complex geologic area, combining slope movements and various fault systems, could be a major concern for the stability of the overlying infrastructure and also the role that a PSI analysis can play in remotely monitoring their behavior over long periods of time.

Crustal Scale Imaging of the Arabia‐Central Iran Collision Boundary Across the Zagros Suture Zone, West of Iran

AbstractWe calculate seismic velocity structure of the north Zagros suture zone, west Iran, to resolve the crustal features at the boundary of Arabian‐Central Iranian collision. We compute teleseismic receiver functions (RFs) for 46 stations along a transect crossing the suture. Through harmonic analysis and inversion of the RF data, we obtain information on the characteristics of the suture zone at depth. The RFs and their harmonics show a low angle NE dipping boundary between the overriding layer and a midcrustal low velocity zone, which corresponds to the suture zone. The overriding high velocity feature (Vs ~ 3.8 km/s) is interpreted as an intermediate depth crustal complex exhumed close to the surface through imbricate thrust faulting and enhanced by crustal buoyancy due to continental underthrusting. Significant anisotropy is found above and below the suture zone: We interpret it in terms of slow‐symmetry‐axis anisotropy and derive clues on the ongoing deformation processes.

Historic Barrel Vaults Undergoing Differential Settlements

ABSTRACT A principal reason of damage in historic masonry vaults consists in relative displacements of the vaults’ abutments. Excluding the case of seismic-induced damage, cracks are often produced by differential settlements generated by the lateral wall instability or soil degradation (e.g., due to stress concentrations, non-uniform soil stratigraphy, flooding phenomena etc.). When dealing with historic vaults, the effects of long-term deformation processes cannot often be linked directly to causes, which may also be unknown. In this article, the effects of differential settlements on historic masonry barrel vaults are investigated. An efficient 3D contact-based model was developed to reproduce experiments on a scaled pointed barrel vault (representative of a typology of late-medieval barrel vaults in Scotland) under non-uniform differential settlement. First, the numerical model is used to simulate the experimental campaign, achieving good agreement in terms of crack pattern (longitudinal shear) and transverse-longitudinal deformation profiles. Then, further analyses are carried out to gain insight on the effects of several plausible uniform and non-uniform settlement patterns on representative historic barrel vaults. Various settlement configurations were analysed and complex failure patterns observed. This study could help analysts in understanding the nature of on-going deformation process in historic masonry vaults and engineers in the design of strengthening strategies.

EXPANDING THE ROLE OF MAGNETIC FIELDS IN RED BLOOD CELLS DEFORMATIONS: DEMONSTRATION OF PARAMAGNETIC AND DIAMAGNETIC FIELDS

Background: The main purpose of this manuscript is to introduce a biophysics based mechanism of the role of magnetism in temporary cell deformation. Biomagnetism: Biomagnetism has been slowly introduced in the medical literature as follows: The diamagnetic nature of blood reported in1936, the recording of organ emissions from the human body also published in the second half of the last century. It could stated that the same way molecular cross talk influence cells signaling, biomagnetic cross talk could also influence cells functions. Introduction: This manuscript introduces the effect of externally applied magnetic fields (diamagnetic or paramagnetic) causing temporary Red Blood Cells (RBCs) deformations. Results from experiments using diamagnetic material such as human blood with the property of repelling an opposing magnetic field; and paramagnetic material such as carbon based graphite or iron particles are reported. Methods: The work herein presented entailed the trapping (between two similar glass slides) of fresh human blood smear with fine exogenous metal, such as iron filings or graphite powder. The technique requires a standard video microscope and an image recording equipment. Provocative placement of ferromagnetic or diamagnetic material in in vitro blood smears preparations were found to exhibit temporary in vivo characteristics quoted Temporary In Vivo Blood Smears (TBIS). Established protocol calls for fresh blood smears to be set-aside 5 minutes prior to staining. During the first 2 minutes of the set-aside period, in a typical monolayer smear, one can observe an initial brief time period of approximately 1’ ± 30” of a drying cycle. Usually, a moisture sheen boundary is seen gradually moving from the slide’s edges towards the center. Results: The main findings include a demonstration that both diamagnetic and pararmagnetic material deform RBCs in a variety of shapes. The paramagnetism (attraction) and diamagnetism of graphite powder induced temporary RBCs deformation in the form of teardrops. For the first time, an ongoing cellular deformation process was captured in video recordings. Conclusions: When in vivo cells in blood smears, such as RBCs, are fronted by exogenous magnetic fields, temporary cell deformation occurs.

Numerical models for planning landslide risk mitigation strategies in iconic but unstable landscapes: The case of Cinque Torri (Dolomites, Italy)

This study deals with the numerical modelling of large deep-seated instability processes affecting the Cinque Torri Group (Dolomites, Italy), which is a UNESCO World heritage site. The aim is to evaluate the role of intrinsic causal factors and to assess the failure mechanism proposed in previous investigations. The geological model is based on topographic, geological and geomechanical surveys, complemented with mineralogical, physical and geotechnical lab analyses for parameters determination. The numerical simulations have been carried out using a Distinct Element Method code on conceptual and site-specific slope models. A series of parametric analyses have been performed to characterize the role of the different predisposing factors potentially related with the ongoing rock spreading: i) load of the rocky pinnacles; ii) lithology, stratigraphy and attitude of the geological units; iii) discontinuity sets in the rock mass; iv) degradation of the mechanical properties. The results of this study suggest that the shear zone initiation is controlled by stress concentration due to the loading of the dolomitic pinnacles, whereas its shape appears to be structurally controlled by the dip-slope attitude of bedding in clay-rich mudstones. The interpretation of the modelling results has provided a better understanding of the ongoing deformation process, which can help in targeting effective and low-impact landslide risk mitigation strategies in this iconic landscape.

The 2015–2016 Ground Displacements of the Shanghai Coastal Area Inferred from a Combined COSMO-SkyMed/Sentinel-1 DInSAR Analysis

In this work, ground deformation of the Shanghai coastal area is inferred by using the multiple-satellite Differential Synthetic Aperture Radar interferometry (DInSAR) approach, also known as the minimum acceleration (MinA) combination algorithm. The MinA technique allows discrimination and time-evolution monitoring of the inherent two-dimensional components (i.e., with respect to east-west and up-down directions) of the ongoing deformation processes. It represents an effective post-processing tool that allows an easy combination of preliminarily-retrieved multiple-satellite Line-Of-Sight-projected displacement time-series, obtained by using one (or more) of the currently available multi-pass DInSAR toolboxes. Specifically, in our work, the well-known small baseline subset (SBAS) algorithm has been exploited to recover LOS deformation time-series from two sets of Synthetic Aperture Radar (SAR) data relevant to the coast of Shanghai, collected from 2014 to 2017 by the COSMO-SkyMed (CSK) and the Sentinel-1A (S1-A) sensors. The achieved results evidence that the Shanghai ocean-reclaimed areas were still subject to residual deformations in 2016, with maximum subsidence rates of about 30 mm/year. Moreover, the investigation has revealed that the detected deformations are predominantly vertical, whereas the east-west deformations are less significant.

Deriving 3-D Time-Series Ground Deformations Induced by Underground Fluid Flows with InSAR: Case Study of Sebei Gas Fields, China

Multi-temporal Interferometric Synthetic Aperture Radar (MT-InSAR) technique has proven to be a powerful tool for the monitoring of time-series ground deformations along the line-of-sight (LOS) direction. However, the one-dimensional (1-D) measurements cannot provide comprehensive information for interpreting the related geo-hazards. Recently, a novel method has been proposed to map the three-dimensional (3-D) deformation associated with underground fluid flows based on single-track InSAR LOS measurements and the deformation modeling associated with the Green’s function. In this study, the method is extended in temporal domain by exploiting the MT-InSAR measurements, and applied for the first time to investigate the 3-D time series deformation over Sebei gas field in Qinghai, Northwest China with 37 Sentinel-1 images acquired during October 2014–July 2017. The estimated 3-D time series deformations provide a more complete view of ongoing deformation processes as compared to the 1-D time series deformations or the 3-D deformation velocities, which is of great importance for assessing the possible geohazards. In addition, the extended method allows for the retrieval of time series of fluid volume changes due to the gas extraction in the Sebei field, which agrees well with those from the PetroChina Qinghai Oilfield Company Yearbooks (PQOCYs). This provides a new way to study the variations of subsurface fluids at unprecedented resolution.

Source mechanisms and faulting analysis of the aftershocks in the Lake Erçek area (Eastern Anatolia, Turkey) during the 2011 Van event (Mw 7.1): Implications for the regional stress field and ongoing deformation processes

In this study, we analysed the source mechanisms and faulting pattern of the aftershocks in the Lake Erçek area, Eastern Anatolia, during the 2011 Van event (Mw 7.1). The fault plane solutions of the aftershocks were used to derive a stress tensor acting around Lake Erçek. The estimated seismological parameters (focal mechanism solutions, stress tensor, and fault focal analyses) were integrated with field surveys and high resolution seismic reflection data so as to better understand the main faulting patterns and deformational features which are the causatives of the observed geomorphological features within and around Lake Erçek. The focal parameters of the 56 aftershocks with magnitude range 3.5≤Mw≤5.5 were obtained using the regional broadband seismic data from the network of Kandilli Observatory and Earthquake Research Institute (KOERI) through the Centroid Moment Tensor (CMT) inversion of locally observed broadband-waves. The implications for the lateral variation of the stress field in the lake area were based on information derived from integration of seismicity and stress tensor inversion results. The major outcomes of our analyses were as follows: (a) The maximum principal stress axis is close to horizontal and aligned in NNW-SSE direction, the minimum compressional stress axis is close to vertical and aligned in an ENE-WSW direction, the intermediate stress axis is close to horizontal and is parallel to the strike of the ruptured fault plane, while the stress ratio is (R=0.6); (b) the lake basin is mostly deformed by dip-slip faults (dips≥45–60°) in and along its margins where the various folds, basement highs with eroded surfaces and several low-angle normal faults are evident; (c) the majority of the aftershocks have reverse-thrust mechanism with some strike-slip components in the Lake Erçek area (N- and S-margins). Strike-slip and normal faulting events are also evident (E- and W-margins, respectively); (d) the normal and reverse mechanisms with some strike-slip component are mostly confined in the upper crust (0–15km), compatible with NS-striking normal and WE-striking reverse faults in the marginal sections of the lake; (e) the WE-striking reverse faulting in the N- and S-margins seems to have the seismic potential to generate destructive earthquakes in the lake area; (f) the tectonic implications of the regional stress field and ongoing deformation processes in the lake are defined and supported by field observations and seismic reflection data. The result of these analyses reveals N-S compression/W-E extension in the lake and also the collisional/compressional regime taking place in a region of Lakes Van and Erçek.

3-D anatomy of an active fault-propagation fold: A multidisciplinary case study from Tsaishi, western Caucasus (Georgia)

Ongoing deformation processes combining fault propagation and folding are the cause of diffuse seismicity in many areas of the world. A detailed understanding of the structural evolution of tectonically active folds is crucial for the evaluation of seismic hazard. This paper proposes an integrated analysis of an active fold, consisting in the development of a 3D model by combination of geomorphological observations, field geological-structural data and seismic reflection sections. Our case study is the Tsaishi anticline, located at the southwestern tip of the Rioni Basin uplifted area, at the foothill of Greater Caucasus (Western Georgia). We recognized that the fold started to form since the beginning of the middle Miocene, although preliminary data suggest the possibility of initial local uplift in the Oligocene. Folding process continues up to nowadays giving rise to a south-vergent anticline, as shown by upwarped late Quaternary river deposits. The fold backlimb is affected by three main back-thrusts, whereas at the foot of the forelimb a main north-dipping thrust comes very close to the surface based on seismic sections. Here, field data show the presence of a 13-km-long fault scarp (or fold scarp). Along the scarp is located the epicenter of the strongest earthquake to date: the MS 6.0 Tsaishi earthquake that struck the area in 1614CE. Based on our results, we propose that the overall structure can be classified as an active fault-propagation fold. The recognition of its very recent growing associated with a major, underlying active fault, represents also a major contribution to the seismic hazard assessment of this populated area.

Decomposing DInSAR Time-Series into 3-D in Combination with GPS in the Case of Low Strain Rates: An Application to the Hyblean Plateau, Sicily, Italy

Differential Interferometric SAR (DInSAR) time-series techniques can be used to derive surface displacement rates with accuracies of 1 mm/year, by measuring the one-dimensional distance change between a satellite and the surface over time. However, the slanted direction of the measurements complicates interpretation of the signal, especially in regions that are subject to multiple deformation processes. The Simultaneous and Integrated Strain Tensor Estimation from Geodetic and Satellite Deformation Measurements (SISTEM) algorithm enables decomposition into a three-dimensional velocity field through joint inversion with GNSS measurements, but has never been applied to interseismic deformation where strain rates are low. Here, we apply SISTEM for the first time to detect tectonic deformation on the Hyblean Foreland Plateau in South-East Sicily. In order to increase the signal-to-noise ratio of the DInSAR data beforehand, we reduce atmospheric InSAR noise using a weather model and combine it with a multi-directional spatial filtering technique. The resultant three-dimensional velocity field allows identification of anthropogenic, as well as tectonic deformation, with sub-centimeter accuracies in areas of sufficient GPS coverage. Our enhanced method allows for a more detailed view of ongoing deformation processes as compared to the single use of either GNSS or DInSAR only and thus is suited to improve assessments of regional seismic hazard.

A slope movement in a complex rock formation: Deformation measurements and DEM modelling

The case history of a deep-seated slope movement in a complex rock formation (Marly-Arenaceous Formation) is analyzed. The movement, monitored for more than 20years, was recognized after the discovery of intense cracking in the concrete lining of a hydraulic tunnel running across the slope. The time history of displacements shows that the ongoing deformation process is essentially a stationary creep phenomenon, also influenced by transient variations in pore pressure distribution. The shearing zone is mainly formed by tectonized clay gouge and is characterized by a mobilized strength close to residual. The slope has been modelled (by DEM approach) as a complex blocky structure defined by several joint sets: bedding planes, inclined and sub-vertical joints. Different hypotheses about the geometry of the slip surface, compatible with field evidences, are discussed as well as their influence on the critical friction angle of the slope. The type of model adopted for rock mass, with continuous or staggered joints, influences the pre-failure deformation mode, but the failure conditions are not as much affected. Finally, the response of DEM models to the increase in water level at the toe of the slope and to the rise of the groundwater table inside the slope has been analyzed. The results of simplified models, without flow calculation, and coupled hydro-mechanical analyses are compared, finding some relevant differences.

A deep seated movement in a marly-arenaceous formation: analysis of slope deformation and pore pressure influence

The case history of a deep-seated slope movement in a complex rock formation (Marly-Arenaceous Formation) is analyzed. The movement, monitored for more than 20 years, was recognized after the discovery of intense cracking in the concrete lining of a hydraulic tunnel running across the slope. The time history of displacements shows that the ongoing deformation process is essentially a stationary creep phenomenon, also influenced by transient variations in pore pressure distribution. The slip surface is formed by a tectonized clay gouge layer and the mobilized shear strength is close to residual. The slope has been modelled (UDEC code) as a complex blocky structure defined by several joint sets: bedding joints, inclined and sub-vertical discontinuities. Different geometries of the slip surface, reasonably varied within the range of hypotheses compatible with field evidences, have limited influence on the limit friction angle of the slip surface. Joint patterns have influence on the deformation mode and minor impact on the mobilized friction angle. The model response is less sensitive to the water level at the slope toe as compared to the rise of groundwater table.

Focal mechanisms in the southern Aegean from temporary seismic networks – implications for the regional stress field and ongoing deformation processes

Abstract. The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 well-located microearthquakes were analysed to produce 540 well-constrained focal mechanisms. They were complemented by another 140 derived by waveform matching of records from larger events. Most of these earthquakes fall into 16 distinct spatial clusters distributed over the southern Aegean region. For each cluster, a stress inversion could be carried out yielding consistent estimates of the stress field and its spatial variation. At crustal levels, the stress field is generally dominated by a steeply dipping compressional principal stress direction except in places where coupling of the subducting slab and overlying plate come into play. Tensional principal stresses are generally subhorizontal. Just behind the forearc, the crust is under arc-parallel tension whereas in the volcanic areas around Kos, Columbo and Astypalea tensional and intermediate stresses are nearly degenerate. Further west and north, in the Santorini–Amorgos graben and in the area of the islands of Mykonos, Andros and Tinos, tensional stresses are significant and point around the NW–SE direction. Very similar stress fields are observed in western Turkey with the tensional axis rotated to NNE–SSW. Intermediate-depth earthquakes below 100 km in the Nisyros region indicate that the Hellenic slab experiences slab-parallel tension at these depths. The direction of tension is close to east–west and thus deviates from the local NW-oriented slab dip presumably owing to the segmentation of the slab. Beneath the Cretan sea, at shallower levels, the slab is under NW–SE compression. Tensional principal stresses in the crust exhibit very good alignment with extensional strain rate principal axes derived from GPS velocities except in volcanic areas, where both appear to be unrelated, and in the forearc where compressional principal stresses are very well aligned with compressional principal strain rates. This finding indicates that, except for volcanic areas, microseismic activity in the southern Aegean is not controlled by small-scale local stresses but rather reflects the regional stress field. The lateral and depth variations of the stress field reflect the various agents that influence tectonics in the Aegean: subduction of the Hellenic slab, incipient collision with continental African lithosphere, roll back of the slab in the southeast, segmentation of the slab, arc volcanism and extension of the Aegean crust.

Segmented seismicity of the M w 6.2 Baladeh earthquake sequence (Alborz Mountains, Iran) revealed from regional moment tensors

The Mw 6.2 Baladeh earthquake occurred on 28 May 2004 in the Alborz Mountains, northern Iran. This earthquake was the first strong shock in this intracontinental orogen for which digital regional broadband data are available. The Baladeh event provides a rare opportunity to study fault geometry and ongoing deformation processes using modern seismological methods. A joint inversion for hypocentres and a velocity model plus a surface-wave group dispersion curve analysis were used to obtain an adapted velocity model, customised for mid- and long-period waveform modelling. Based on the new velocity model, regional waveform data of the mainshock and larger aftershocks (Mw ≥3.3) were inverted for moment tensors. For the Baladeh mainshock, this included inversion for kinematic parameters. All analysed earthquakes show dominant thrust mechanisms at depths between 14 and 26 km, with NW–SE striking fault planes. The mainshock ruptured a 28° south-dipping area of 24 × 21 km along a north-easterly direction. The rupture plane of the mainshock does not coincide with the aftershock distribution, neither in map view nor with respect to depth. The considered aftershocks form two main clusters. The eastern cluster is associated with the mainshock. The western cluster does not appear to be connected with the rupture plane of the mainshock but, instead, indicates a second activated fault plane dipping at 85° towards the north.

Evidence of large scale deformation patterns from GPS data in the Italian subduction boundary

We present the velocity field in Italy derived from over 300 continuous GPS stations operated in the 1998–2009 time span. The GPS network maps the whole country with a mean inter-site distance of about 50 km and provides a valuable source of data to study the ongoing deformation processes in the central Mediterranean. The estimated horizontal and vertical velocity fields show major significant features and also less known second-order kinematic features. A general uplift characterizes the whole Apennines and Alpine belts that follow the topographic ridge, whereas the Po Plain shows a gradually increasing subsidence from west to east. The Apennines belt displays a distinctive extension (50–80 10 − 9 yr − 1 ) while compressive tectonic regimes characterize northern Sicily, eastern Alps and the northeast front of the northern Apennines (25–50 10 − 9 yr − 1 ). Second-order deformation patterns, on large scale wavelength (~ 100 km) have been detected on the accretionary prism of central and southern Apennines that are highly correlated with other geophysical data (Vp anomalies, seismic anisotropy, etc.) and related to deep rooted sections (70–100 km), marked by different subduction regimes. Apparently at this scale-length the observed deformations are governed by the lithosphere as a whole. We interpret these deformations as a result of different subduction mechanisms, such as variations of the subduction rollback velocity affecting different segments of the subduction zone and/or to mantle flows in proximity of the slab edges. Further south, in central-southern Sicily, we detect a contraction of (− 1.1 ± 0.2) mm/yr that probably accommodates part of the Africa–Eurasia convergence on the outer thrust front of the Apennines–Maghrebides belt. This hypothesis agrees with an independent analysis of the seismicity associated to the Sicilian Basal Thrust, thought to be still active. The ITRF2005 estimates of the new GPS velocity field are available also in SINEX format as supplementary file S1.